1. Field of the Present Invention
The present invention relates broadly to a powdered metal ("P/M") admixture and a method or process of forming a P/M admixture. More particularly, the invention relates to a P/M multi-phase bronze material for bearings/bushings and structural parts and a process of forming a PIM multi-phase bronze material for bearings/bushings and structural parts. The resultant parts are characterized as being lighter weight, stronger parts at net shape than conventional cast metal bronze material parts.
2. Description of the Background
P/M technology is a highly developed method of manufacturing reliable ferrous and nonferrous parts. PIM technology is well known to persons skilled in the art and typically consists of three (3) basic steps of mixing elemental or alloy powders, compacting the mixture into a die and sintering the resultant shapes in a controlled atmosphere furnace to bond the particles metallurgically. Those steps may be followed by optional secondary operations, one of which is oil impregnation.
P/M technology is a relatively inexpensive process that results in high volume production. Basically a "chipless" metalworking process, powdered metallurgy typically uses more than 97% of the starting raw material in the finished part. Also, only minor, if any, machining is required on the P/M parts and they maintain close dimensional tolerances. Because of this, P/M processing is an energy and materials conserving process. It is well known in the art to produce many different products comprised of many different metals through P/M technology, examples of which are bronze bearings/bushings and structural parts.
P/M bronze parts are commonly produced by blending pre-alloyed metals into a mixture, compacting the mixture, and sintering the product in a 90/10 atmospheric mix of nitrogen/dissociated ammonia ("N.sub.2 /A") at 1625-1650.degree. F. (885-899.degree. C.). P/M technology is very product specific and each variable, such as sintering time, temperature, atmospheric conditions, compacting pressure, etc., is dependent upon the starting materials to be used and the desired resultant product characteristics.
Most properties of a P/M metal part are closely related to its final density. This density is the mass per unit volume of the part expressed in grams per cubic centimeter (g/cm.sup.3). Density is also expressed as relative density, which is defined as the ratio of a P/M part's density to that of its pore-free equivalent. As with wrought and cast metals, chemical composition of P/M parts strongly influences the mechanical properties of strength, ductility, hardness (apparent) and particle hardness.
It is well known in the art to use the concept of minimum values of properties for P/M materials. These values, such as oil content and radial crushing force, may be used in designing for a P/M bearing application. It is seen as an advantage of the P/M process that equivalent properties can oftentimes be developed by varying chemical composition, particle configuration, density and/or processing techniques. The grade of material that is selected is determinant upon the design of the part and its end use, including dimensional tolerances, as well as other characteristics such as density, porosity, compressive strength, corrosion resistance, oil content, surface finish, and any other pertinent requirements. While P/M technology is employed to produce a wide range of parts comprised of a variety of raw materials; some materials and resultant parts currently may not be produced using conventionally known P/M technology processes.
Typically, aluminum bronze bearings/bushings and structural parts are cast-metal foundry products. These cast products are desirable because they are characterized as having a high degree of strength. However, they are also characterized as being fully dense, that is, they have no porosity, therefore they do not take favorably to oil impregnation and are also quite weighty. Another disadvantage of this process is that casting and then machining these parts often results in a relatively low volume production, as this process is often time intensive. Additionally, casting and then machining parts may be costly as well. Therefore, it is often preferable to form parts through P/M technology rather than through casting if at all possible.
Yet, forming aluminum bronze products through conventionally known P/M technology has not produced a product with the desired characteristics, for example, a specified high degree of strength. Therefore, there is a need in the art to produce aluminum bronze multi-phase parts that are lighter weight with increased strength, and that have fully dense properties at a lower density to allow for oil impregnation, through a relatively inexpensive process that results in high volume production.